The study suggests that this unpredictability is inherent in a system where people's tastes interact, Salganik says.

The researchers published their results in Friday's issue of the journal Science.

The Paradox of Success

The researchers were motivated to study music downloading to understand a paradox about how things become popular, Salganik said.

Take the first Harry Potter novel. "It has been tremendously successful and been translated into more than 60 languages," Salganik said.

"That would make you believe Harry Potter is somehow different from all the other books that didn't have nearly this level of success," he said.

"But this difference wasn't at all clear ahead of time," Salganik said. Many publishers, thinking it wouldn't succeed, rejected the book.

The same phenomenon shows up with music and movies.

"A lot of this unpredictability is invisible" to the public, Salganik said, because it never sees the things that fail.

One of the most spectacular bombs, he points out, was Irish singer Carly Hennessy. Record company MCA threw 2.2 million U.S. dollars into producing and promoting her album Ultimate High. It sold only 378 copies in the first three months.

"I went online and bought her CD for a penny," Salganik said. "It actually doesn't sound that much different from a lot of other things you hear. It just seemed like she had an unlucky break."

Testing, Testing, 1, 2

The Music Lab experiment was not quite like the real world, Salganik says. Most of the listeners were teens, and there were no celebrity endorsements or advertising to affect the results.

Yet the experiment's results are similar to those from mathematical models of the success of movies, said economist David Walls of Canada's University of Calgary. He and economist Art De Vany of University of California, Irvine, have modeled how people tell each other about movies.

"When [people] see a movie they like, they make a discovery, and they tell their friends about it," Walls and De Vany wrote in a 1996 article in The Economic Journal.

This creates a feedback that unpredictably leads some movies to break out and others to nosedive, Walls said.

Sociologist Michael Macy of Cornell University finds the Music Lab study convincing.

"They're showing that it's very hard to predict which songs will become popular," he said. "That's not just because we don't know what peoples' tastes are," Macy said.

"It's because random fluctuations early on in the reports about what's popular become exaggerated due to the social influence."

However, economist Stan Liebowitz, at the University of Texas in Dallas, is skeptical about how much the study proves.

To improve the data, he suggests tricking some of the listeners by giving them fake data so that the songs that were actually most popular would appear to be the least popular, and vice versa.